1. Field of the Invention
This invention relates to multiplexed telecommunication systems, especially to time division multiplexed telecommunication systems, such as pulse code modulation (PCM) telephone networks in which a plurality of stations are interconnected in tandem by looped transmission lines.
2. Description of the Prior Art
The system of the above-noted type is used, for example, to communicate between a main station and a plurality of local railway stations.
A plurality of such stations on the loop transmission lines communicate with one another through the individual pulse multiplexing and demultiplexing circuits which are provided in each station.
That is, signals from a first station are transmitted in a respective time slot through a multiplexing circuit in the first station to a second station and are received at a demultiplexing circuit in the second station. On the other hand, signals from the second station are transmitted through a multiplexing circuit in the second station and are received at a demultiplexing circuit in the first station.
The conventional telecommunication system of this type has two individually looped transmission lines in order to maintain the reliability thereof. Thus, if a break or fault occurs in the first transmission line, termed a normal using line, each station on the looped line is immediately connected to the second transmission line which is referred to as a stand-by line, and the signals are transferred back to the normal using line, i.e. the conventional system incorporates bidirectional data transmission lines by use of the using line and the stand-by line. In order to construct the system of this type, each station must have four individual multiplexing circuits and also four individual demultiplexing circuits.
Since the multiplexing and demultiplexing circuits are expensive, especially for use in an optical communicaton system, the conventional system as outlined above becomes very expensive.